The invention is based on a hydraulic brake system as defined hereinafter.
A dual-circuit brake system for a two-axle motor vehicle of this kind has already been proposed (German Patent Application P 40 04 270.7, FIG. 7, see related patent application), in which a high-pressure pump, as a function of a pressure pickup connected to a brake line, aspirates pressure fluid from a pressure fluid supply tank of the master brake cylinder, with shutoff valves closed, and feeds it into the brake lines of both brake circuits. With the aid of an electronic control unit, the brake pressure in the brake circuits, each of which is assigned to one vehicle axle, is controlled in accordance with the so-called ideal brake force distribution. Moreover, in this brake system, locking of the vehicle wheels upon braking or spinning of driven vehicle wheels is avoided by making an anti-lock and traction control system come into play, which in phases of pressure reduction draws brake fluid out of the wheel brake and returns it to the pressure fluid supply tank or in phases of pressure buildup feeds it into the brake line. The excess pressure fluid pumped by the high-pressure pump, which is driven continuously during braking and aspirates from the pressure fluid supply container of the master brake cylinder is diverted to the pressure fluid supply container via a pressure limiting valve. This is unfavorable, however, because the pumping operation produces undesirable heating of the pressure fluid and of the pump drive motor.